Semiconductor nanoparticles whose particle sizes are 10 nm or less are located in the transition region between bulk semiconductor crystals and molecules. Their physicochemical properties are therefore different from both bulk semiconductor crystals and molecules. In this region, due to the quantum-size effect, the energy gap of semiconductor nanoparticles increases as their particle sizes decrease. In addition, the degeneration of the energy band that is observed in bulk semiconductors is removed and the orbits are dispersed. As a result, the lower-end of the conduction band is shifted to the negative side and the upper-end of the valence band is shifted to the positive side.
Semiconductor nanoparticles exhibit spectra with peaks whose full widths at half maximum are relatively narrow. Accordingly, particle size regulation of semiconductor nanoparticles enables the development of various reagents which exhibit spectra whose full widths at half maximum are narrow. This enables the multi-color analyses in, for example, the detection and the imaging of biopolymers. Compared to common organic pigments, semiconductor nanoparticles are more durable and fade less.
Semiconductor nanoparticles can be easily prepared by dissolving equimolar amounts of precursors of Cd and X (X being S, Se or Te). This is also true for the manufacturing of, for example, CdSe, ZnS, ZnSe, HgS, HgSe, PbS, or PbSe.
Semiconductor nanoparticles have drawn attention since they emit strong fluorescences whose full widths at half maximum are narrow. Thus, various fluorescent colors can be created, and their future applications can be nearly infinite. However, the semiconductor nanoparticles obtained by the above method exhibit a wide distribution of particle sizes and therefore cannot provide the full advantage of the properties of semiconductor nanoparticles. Attempts have been made to attain a monodisperse distribution by using chemical techniques to precisely separate and extract only the semiconductor nanoparticles of a specific particle size from semiconductor nanoparticles having a wide distribution of particle sizes immediately after preparation. The attempts to attain a monodispersed distribution of particle sizes that have been reported so far include: separation by electrophoresis that utilizes variation in the surface charge of nanoparticles depending on their particle sizes; exclusion chromatography that utilizes differences in retention time due to different particle sizes; size-selective precipitation that utilizes differences in dispersibility in an organic solvent due to differences in particle sizes; and size-selective photoetching that utilizes the oxidative dissolution of a metal chalcogenide semiconductor in the presence of dissolved oxygen when irradiated with light.
The aforementioned production methods are carried out by preparing semiconductor nanoparticles having a wide distribution of particle sizes and then regulating and selecting the particles sizes. An example of a method for preparing previously monodispersed nanoparticles is the reversed micelle method that utilizes amphiphilic molecules. In this method, reversed micelles are formed in a non-polar solvent. The inside of the reversed micelle is regarded as a reaction field, and the size of the reaction field is regulated, thereby sorting nanoparticles into uniform particle sizes. This has been the most commonly employed method up to the present, and is the easiest method of particle size preparation. When monodispersion by the reversed micelle method is carried out, the distribution of particle sizes is approximately several dozen percentage points. Semiconductor nanoparticles emit fluorescence upon the application of excitation light, and the wavelength of this fluorescence is determined by the particle size. Specifically, a fluorescence spectrum whose full width at half maximum (FWHM) is narrow cannot be obtained where there is a wide distribution of particle sizes. Accordingly, the distribution of particle sizes should be further reduced in order to prepare semiconductor nanoparticles which emit relatively monochromatic fluorescences.
In contrast, size-selective photoetching for attaining monodispersion of particle sizes that utilizes the oxidative dissolution of a metal chalcogenide semiconductor in the presence of dissolved oxygen when irradiated with light has been heretofore used as a method for preparing monodispersed semiconductor nanoparticles having a wide distribution of particle sizes. In this method, particle size selection or the like is unnecessary, and particle sizes can be monodispersed in a bulk solution. When the semiconductor nanoparticles obtained by this method are irradiated with light having a wavelength of 476.5 nm, the average particle size is 3.2 nm and the standard deviation is 0.19 nm. These semiconductor nanoparticles exhibit a very narrow distribution of particle sizes, i.e., the standard deviation is approximately 6% of the average particle size. This indicates that the distribution of particle sizes is very close to the monodispersed state.
Previous methods of production of semiconductor nanoparticles required a stabilizer, and it was difficult to combine this production method with the latter monodispersion into a sequential method.
Therefore, an object of the present invention is to develop an effective sequential method for producing monodispersed semiconductor nanoparticles.